He has now played 81 games in MLB, all of them before his 23rd birthday. He has produced 2.1 fWAR in those games.
ZIPS is now convinced he's an above average starter true talent and as you said you can squint and see a future where a 10 WAR season -- say around 2021 -- isn't out of the question. If he slashes 280/400/530 and is plus 10 runs at 2B he's getting close, that would probably be about 8 fWAR.
He's a fun guy to dream on, you can see why he was #1 overall for a minute.